Lyme disease is the most prevalent arthropod bone infection in the United States. The disease, caused by the spirochete Borrelia burgdorferi, is a multiple systemic disorder with various clinical manifestations. In the United States, one of the major manifestations of the acute disease is Lyme arthritis. Approximately 60% of untreated patients develop intermittent attacks of monoarticular or oligoarticular arthritis, primarily in large joints such as knees. Previous studies indicate that B. burgdorferi is highly invasive. These spirochetes traverse the intercellular matrix, penetrate the vascular endothelial cell lining, and finally invade the joints after being deposited in the skin following a tick bite. However, the mechanisms involved in this invasive process is still unknown. The present proposal focuses on the motility of B. burgdorferi, and its role in the disease process. Only recently have the tools for gene targeting been developed for B. burgdorferi, In addition, an understanding of its complex motility is at a very early stage. I hypothesize that the flagellar genes fliG2 and fliG1 play critical but different roles in B. burgdorferi motility. Preliminary results suggest that a fliG1 null mutant continuously swims but is unable to translate (i.e. show displacement), and a fliG2 null mutant is completely non-motile. I also hypothesize that fliG1 functions to coordinate the rotation of the motility organelles, the periplasmic flagella, that allow for directed cell movement. To test these hypotheses, I will characterize these mutants in detail. Green fluorescent protein fusions, and the yeast two hybrid system, will be used to analyze the function of FliG1 and FliG2 in depth. The information obtained should yield a better understanding of molecular mechanisms of B. burgdorferi motility. Second, I hypothesize that motility is a virulence factor. To test this hypothesis, I will target these two genes in a difficult to manipulate virulent strain, analyze the resultant mutants in depth, and test the virulence of these mutants by the mouse model of Lyme disease. I predict that these two mutants will be less virulent than the parental strain. The results obtained will yield critical information on B. burgdorferi motility and and its relationship to virulence. These results could lead to new means of disease prevention and treatment.